Near-field inductive links have been extensively used in conventional methods for short-range data and power transmission. An inductive link between two magnetically-coupled coils is now one of the most common methods to wirelessly send power and data from the external world to an Implantable Medical Device (“IMD”) that requires relatively high data transmission bandwidth. Conventional inductive links include two adjacently coupled LC tank resonant circuits—one on a receiver-side and one on a transmitter side. By tuning these LC tank circuits at a wireless carrier frequency, the amplitude of a transmitted signal on the receiver-side can be increased significantly while attenuating the out-of-bound interference.
There are numerous applications for using inductive links to transmit and receive data. For example, IMDs that can use inductive links to receive data and power include, but are not limited to, neuromuscular stimulators, cochlear implants, visual prostheses, and the like. Further, applications seeking to avoid the use of batteries due to size, cost, and lifetime constraints can highly benefit from inductive coupling systems and methods. Examples of these applications include, but are not limited to, Radio-Frequency Identification (“RFID”), contactless smartcards, wireless Microelectronic Mechanical Systems (“MEMS”), and the like. Achieving high power transmission efficiency, high data transmission bandwidth, and small size while maintaining robustness and a low Bit-Error-Rate (“BER”) against impediments, such as external interference, supply ripple, load changes, internal digital switching noise, and coupling variations due to vibrations and coils misalignments, are some of the major challenges in the design of conventional inductive coupling systems.
Some biomedical implants, particularly those that interface with the Central Nervous System (“CNS”), such as cochlear and visual prostheses, need relatively large amounts of data to simultaneously interface with a large number of neurons through multiple channels. In some instances, a minimum of 625-1000 pixels are needed in a visual prosthesis to enable a patient to read text with large fonts. Every stimulation command in such prostheses can require ten bits for addressing the stimulating sites, six to eight bits for stimulation pulse amplitude levels, and two to four bits for polarity, parity-checking, and sequencing. This would suggest at least 20-bits per command frame are required for site selection and stimulus amplitude information. Considering that it might be necessary to stimulate electrodes at rates up to 200 Hz each (for physiological reasons) and there is a need for up to four commands per biphasic-bipolar stimulation pulse in some microstimulator architectures, then raster scanning all 625 sites at this rate requires a serial data bit stream of 625-sites×20-bits×4-commands×200-frames=10 Mbps. The fact that all the electrodes might not need to be refreshed at all times, significantly reduces the required data rate. However, it is clear that a high data transmission bandwidth is needed for wireless implantable microstimulators that interface with the CNS.
Data transmission techniques in conventional systems have mostly been carrier based. In broadband wireless communications, such as the IEEE-802.11a standard for wireless LAN application, data-rates as high as 54 Mbps have been achieved at the expense of increasing the carrier frequency up to 5.8 GHz, which yields a data-rate to carrier-frequency ratio of only 0.93%. In other words, each data bit is carried by 107.4 carrier cycles. Such high carrier frequencies, however, are impractical for use in IMDs. The maximum carrier frequency for biomedical implants is limited to a few tens of MHz (generally 20˜30 MHz) due to the coupled coils' self-resonance frequency, increased power loss in the power transmission circuitry at higher carrier frequencies, and excessive power dissipation in the tissue, which increases as the carrier frequency squared (fr2). Therefore, a goal of conventional inductive coupling systems and methods is to transmit/receive each data bit with a minimum number of carrier cycles in order to maximize the data-rate to carrier-frequency ratio and minimize power consumption.
Implantable devices that record the neural activity and send the information out of the body also need wide bandwidth data links. This is because neural signals have a wide bandwidth (0.1 Hz˜10 kHz) and recording from a large number of electrodes generates a large volume of data. Conventional systems have employed different modulation schemes for transmitting data outside the body. A goal of these systems is to use modulation circuitry that is very low power, capable of handling high data rates, and very small. Also, because power and data are often transmitted simultaneously, the data link must be robust in the presence of the power carrier interference, which can be orders of magnitude larger than the data carrier.
Several modulation schemes have been used in conventional systems for data transmission via an inductive link, including Amplitude Shift Keying (“ASK”), On-Off Keying (“OOK”), Frequency Shift Keying (“FSK”), Phase Shift Keying (“PSK”), and Pulse Position Modulation (“PPM”). ASK has been commonly used in conventional systems and methods because of its simple modulation and demodulation circuitry. This method of modulation represents digital data as variations in the amplitude of a carrier signal. ASK, however, is not robust against coupling variations and faces major limitations for high-bandwidth data transmission. High-bandwidth ASK needs high order filters with sharp cut-off frequencies. This requires large capacitors that cannot be easily integrated in the low-frequency end of RF applications.
A remedy that has been proposed is the so-called suspended carrier modulation or On-Off Keying (“OOK”), which is a simple form of ASK that represents digital data as the presence or absence of a carrier wave. OOK boosts the modulation index up to 100% (turning the carrier on and off) to achieve high data rates with low-order integrated filters at the expense of an average 50% reduction in the carrier power. Even with OOK, however, achieving high data-rates is challenging, and data rates are usually less than 10% of the carrier frequency.
FSK and PSK operate by modulating data in the frequency or phase of a sinusoidal carrier wave, respectfully. A limitation with FSK is that it occupies a relatively wide bandwidth (>5 MHz). PSK has fewer limitations compared to ASK, OOK, and FSK. However, the dependence of all of these methods on a carrier signal results in high power consumption, particularly on the transmitter side of the transmissions system.
For wideband data transmission from IMDs to the outside of the body (referred to as the uplink), most conventional systems have employed active back telemetry circuits, such a Voltage Controlled Oscillators (“VCO”), that utilize similar carrier based modulation techniques in the far-field domain. This has been done despite the fact that the external receiver antenna can be placed in a patch right across the skin. As a result, the RF transmitter is one of the key power consuming blocks in such IMDs. An alternative method used in conventional systems is passive back telemetry using Load Shift Keying (“LSK”), which is abundantly used in RFID tags for data rates up to 0.5 Mbps. LSK is limited because of its requirement for strong coupling between coils, a switch across the IMD power coil, and an ASK receiver outside of the body. Although the switch is relatively simple to implement and consumes little power, the switch entirely shuts off the power transfer to the IMD when it is closed. Hence, a fundamental problem with LSK is reducing the IMD received power by the switching duty cycle, which is not desired in high performance IMDs.
In 2002, the U.S. FCC issued a ruling deregulating the use of Ultra-Wide Band (“UWB”) for communications. A variation of UWB, known as Impulse Radio (“IR”)-UWB, in which data is transmitted via sub-nanosecond pulses through wideband antennas, soon became popular in conventional systems for short-range low applications, such as Body Area Networks (“BAN”). Conventional IR-UWB systems are able to consume small amounts of power because they are carrier-less, which means they do not require any continuously on power-consuming, high-frequency oscillators or frequency stabilization circuitry. These characteristics make IR-UWB look like an attractive choice for the uplink in IMDs. The caveat, however, is that the ordinary IR-UWB, which is intended for far-field interactions in the 3.1˜10.6 GHz band, is highly absorbed in water; thus, it cannot penetrate or pass through a tissue volume conductor.
Additionally, in UWB, modulation schemes such as OOK and PPM have been used in conventional systems by coding data in the presence and/or location of the pulses. One of the benefits of UWB is the use of discrete pulses for data transmission instead of continuous carrier signals. This eliminates the use for power consuming components such as oscillators, PLLs, and mixers, and, therefore, contributes to the design of low power and low complexity transmitters. There are, however, problems with using these UWB methods in applications where a power carrier is present. The power carrier interference can dominate the data carrier on the receiver side, such that data recovery becomes nearly impossible.
Several conventional systems have adopted the idea of a carrier-less wireless link from the IR-UWB and applied it to inductive links in the near-field domain. These methods use sharp pulses to transfer data in applications, such as chip stacking, multimedia, and BAN. In most of these methods, to achieve high data rates, the Self Resonance Frequencies (“SRF”) of both the Transmitter (“Tx”) and Receiver (“Rx”) coils are kept quite large to allow the high frequency components of the sharp data carrying pulses to effectively pass through the inductive link and reconstruct the pulse on the Rx side. It can be shown that when a simple Gaussian pulse with the width of passes through an inductive link with LC tank circuits on the Rx and Tx sides, the pulse is differentiated by the inductive coupling between the coils and its fundamental frequency shifts from DC to fP=√{square root over (2)}/πtpw. As a general rule, the inductive link bandwidth should be kept above 2fP to limit undesired Inter-Symbol Interference (“ISI”). Otherwise, if the inductive link bandwidth is not wide enough, it significantly attenuates the higher harmonic components of the sharp transmitted pulse. This results in ringing on the Rx side that extends well beyond the designated bit period. This will either increase the ISI and BER or lead to data rate reduction—both of which are undesirable.
Maintaining high SRF in 100's of MHz range in implanted coils, which are used in IMDs, is not quite feasible because the coils' dimensions, inductance, an separation are often much larger and their parasitic resistance is much lower than that of the on-chip coils used in chip-to-chip communication. Due to high conductivity of the tissue volume conductor, there is also significantly more parasitic capacitance around the IMD coils that are implanted or attached to the body compared to those operating in the air (BAN). Therefore, many of these conventional methods are inapplicable in IMDs.
A possible solution to the bandwidth limitation is lowering the coils' quality factor, Q, by adding series or parallel resistors to the coils. Unlike coils used in IMDs, on-chip coils inevitably have low Qs due to their high parasitic resistance. Low Q, however, has the undesirable effect of decreasing the range of the inductive link, i.e. the maximum coil separation. This occurs because the amplitude of the received signal decreases and the noise and interference due to lowering the receiver selectivity increases, which, consequently, degrades the Rx Signal-to-Noise Ratio (“SNR”).
Therefore, there is a desire for systems and methods that take advantage of the low power and low transmitter-complexity properties of conventional UWB methods as well as interference rejection properties of the conventional resonance based methods. Further, there is a desire for systems and methods that allow coils to maintain their high Q, filter out undesired sources of interference, increase the inductive link voltage gain, and maximize the SNR at the Rx.